In FCC units, most of the propylene leaves the FCC fractionator overhead receiver in the vapor phase, from which it is recovered in downstream FCC gas concentration facilities. By virtue of the mixing which takes place between liquid and vapor phases in conventional overhead condensing and receiving steps, the liquid phase leaving the receiver carries with it a considerable amount of propylene and light ends. The liquid is the unstabilized gasoline which is used as the lean oil for the primary absorber of FCC gas concentration unit. The presence of light ends and propylene in the unstabilized gasoline increases the load on the primary absorber and this will result in losses of propylene to fuel gas and less propylene recovery. Increased loss is experienced when the unit operating pressures at main column receiver are increased in view of the fact that the liquid-vapor equilibrium at higher pressures favors more propylene in the liquid phase. The same concept for minimizing propylene in unstabilized gasoline before routing to primary absorber was discovered by UOP and discussed in U.S. Pat. No. 3,893,905.
The equipment requirements discussed in U.S. Pat. No. 3,893,905 is a modification in the design of FCC fractionator overhead receiver condensers. This produce complexity in the design and not possible for existing FCC units. The present invention uses a simpler equipment design which can be used for existing FCC units.
In the traditional process, the product mixture from FCC main column overhead comprising gasoline, LPG, and fuel gas are first condensed and gravity separated to produce unstabilized gasoline, which is subsequently used in the primary absorber to absorb propylene and LPG from fuel gas. However, high recoveries of propylene become more difficult or uneconomical using traditional processes for recovery due to the presence of light ends and propylene in the unstabilized gasoline. Propylene in unstabilized gasoline can range from about 2 mol % to about 5 mol % or above.
The present invention reduces the amount of propylene and LPG lost in FCC units operating either at lower or higher pressures. In the embodiments of this invention a primary absorber feed flash drum decreases the propylene and light ends in unstabilized gasoline and when it is routed to the primary absorber more propylene and LPG are recovered.